This invention relates to a method for hoisting an item by means of a crane. More particularly, it relates to a method for hoisting an item at sea with a hoisting device, comprising:                moving the item between different a first height and a second height        alternately supporting the item with a first hoisting rope and then a second hoisting rope;        arranging the first hoisting rope and the second hoisting rope to extend in parallel along at least a portion of the distance between the item and the hoisting device; and        releasably connecting the first hoisting rope to the second hoisting rope. Embodiments of the invention also comprise devices for practising the methods.        
During hoisting operations at sea where heavy items having weights in the order of several hundred tonnes are to be disposed on the seabed, for example, the availability of steel ropes having sufficient combined strength and length has become a limiting factor for the size of items that can be handled. The seabed may be located several kilometres below sea level, and the weight of the steel rope therefore becomes significant.
It may therefore be necessary to use fibre ropes to allow the largest items to be submerged into deep waters.
The use of fibre ropes for operations of this type requires consideration of conditions not normally being limiting when using steel ropes. For example, the effective life of a fibre rope comprising a significant proportion of carbon fibre depends directly on the number of load-related flexures that the fibre rope is exposed to.
Oftentimes hoisting operations of this type are heave-compensated, and the lifting rope will therefore be continuously reeled in and out from the winch due to the heave motion of the lifting vessel. Even if the item being lifted is stationary relative to the seabed, the lifting rope will still be reeled in and out, whereby the effective life of a fibre rope is reduced relatively fast.
Moreover, fibre ropes do not possess sufficient shape stability for allowing them to be reeled in several layers onto a drum when full lifting power is applied to the fibre rope.
Similar to a belt, it is known to wind a fibre rope several turns around two parallel drums (traction winch) before pulling the fibre rope onto a storage reel, often in several layers, whilst using a relatively small force.
Such a solution, however, causes the fibre rope to be exposed to flexing every time it enters a drum and every time it leaves the drum. If the fibre rope is wound five parallel turns around the two drums, the rope will be continuously exposed to repeated flexing at ten places, which reduces the effective life to an unacceptable extent.
Fibre ropes designed to transmit the lifting power to a reel are relatively expensive.
It is known to pay out a relatively long rope, which carries a load, by means of a shorter rope. For example, U.S. Patent Application Pub. No. 2005/0191165 describes a method wherein a first length of fibre rope is paid out by means of a lifting wire on a vessel, and wherein the connection of the lifting wire to the fibre rope is moved before a next length of fibre rope, which is connected to the first length of fibre rope, is paid out by means of the lifting wire. The pay-out takes place in a fixed position relative to the vessel.
The object of the invention is to remedy or reduce at least one of the disadvantages of the prior art.